Cross-Covariance Matrices in the Billoir Vertex Fit
Description
In this white paper we derive \begin{itemize} \item the 4x4 covariance matrix of the vertex position, \item the 4x3 cross-covariance matrix between the vertex position and the \(i\)-th particle momentum, \item and the 3x3 cross-covariance matrix between the \(i\)-th and the \(j\)-th particle momentum. \end{itemize}